Mechanism of graphitization and optical degradation of CVD diamond films by rapid heating treatment

Polycrystalline CVD free-standing diamond films have been heat treated through a rapid heating process using a direct current (DC) arc plasma jet system within the temperature range of 1500-2000 degrees C. The optical transmission of the diamond film decreases when the treatment temperature exceeds 1500 degrees C. When the diamond film was annealed to 1800 degrees C for just 1 min, the infrared (IR) transmittance at lambda = 10.6 mu m of the film was significantly decreased from 60.5% to 4.0%, due to the transformation from diamond to graphite. Further increase of the temperature to 2000 degrees C leads to complete loss of optical transmittance in only 30 s, due to the detrimental occurrence of graphitization. XRD patterns indicate that graphite ultimately grows preferentially along the (0002) plane by forming disordered or turbostratic graphite as intermediate product. Raman spectra and SEM images verify that at 1500-1850 degrees C graphitic carbon transforms from the diamond surface in the form of protruding submicron crystallites, which were still containing a number of sp(3) C-C bonds. In addition to graphitization on the external surface, the diamond-graphite phase transition also occurs along the grain boundaries and the calculated activation energy of the grain boundary graphitization process is 276 kJ.mol(-1) for the CVD diamond. (C) 2016 Elsevier B.V. All rights reserved.